Hydrogen peroxide, as a strong oxidizing agent, has been commonly used as a bleaching agent of pulp used in the paper industry and to bleach textiles. Hydrogen peroxide is particularly suited to bleaching applications because, unlike most other bleaching chemicals, it lends itself well to continuous processing, has no severe toxicity problems, is virtually noncorrosive and causes a minimum of effluent problems.
Hydrogen peroxide has been found to be particularly useful in the bleaching of mechanical pulps where most of the lignin is retained in the pulp and where high brightness values are not critical. Bleached groundwood pulp of relatively high quality, with respect to cleanliness, brightness and brightness stability result with hydrogen peroxide bleaching. It has been found that the most efficient bleaching action is obtained when the proportion of reagents to pulp is sufficient to achieve a residual hydrogen peroxide content in the spent liquor of about 25% to 50% of that originally present. This represents a substantial concentration of hydrogen peroxide in the spent liquor and recovery and re-utilization of this hydrogen peroxide is becoming increasingly important to the pulp and paper industry.
Such recovery and re-utilization would promote increased efficiency of hydrogen peroxide use, reduce the cost of waste effluent treatment and reduce the flow of waste hydrogen peroxide discharged into the environment, or the chemical costs of neutralizing it before discharge.
In the past steps have been taken to use this spent liquor again by contacting it with fresh unbleached pulp. This achieves some brightening benefit but the presence of colored constituents in this partially active spent liquor makes it far less effective in its bleaching ability as compared to an equivalent mass of fresh, colourless hydrogen peroxide. Consequently, there is a need for a simple, inexpensive method for separating colored constituents from an aqueous hydrogen peroxide solution to enable the hydrogen peroxide to be used more efficiently in the bleaching process. As well, other processes employing hydrogen peroxide may benefit from this method including processes relating to the production and purification of hydrogen peroxide for sale and use as such.
As well, a significant number of chemical pulp bleach plants have adopted a final peroxide bleaching stage. The hydrogen peroxide stage permits stabilization of the brightness value to a maximum degree and reduces the discharge of harmful chorine derivatives into the environment. The present invention may also be used to permit re-use of this spent hydrogen peroxide solution.
In the past several methods have been devised to separate aqueous hydrogen peroxide from these colored constituents, including: